This invention relates generally to a method for measuring ion concentration of a sample solution by detecting a potential difference generated between an ion electrode and a reference electrode which are dipped in the sample solution, and more particularly to a method for measuring the ion concentration while calibrating or compensating variations in a reference point and a span with the aid of at least two standard solutions having known ion concentrations.
The problems encountered at the ion concentration measurement are a drift of the reference point and a deviation of the span due to disturbances such as variations of temperature, humidity and the like, and fluctuations of a voltage supply source. Such problems require calibrations or compensations of the reference point and span. In known methods the calibrations are effected with the aid of at least two standard solutions having known ion concentrations in such a manner as will be explained hereinafter with reference to FIG. 1.
FIG. 1 is a block diagram showing an ebodiment of a known ion concentration measuring apparatus. The apparatus comprises an ion electrode or a measuring electrode 1 and a reference electrode 2 which are so constructed to respond selectively to given ions to be measured. When the electrodes 1 and 2 are dipped into a solution with the given ions contained in a vessel 3, there will be generated a potential at respective electrode and a difference between these potentials is a measure of an ion concentration of the solution. This potential difference is amplified by an amplifier 4 and then is applied to an indicator 5 where the potential difference is displayed in the form of the ion concentration. Prior to the measurement, the reference point and the span have to be calibrated with the aid of an adjuster 6 inserted between the amplifier 4 and the indicator 5. Usually the adjuster 6 comprises a stage for shifting a signal level and a stage for adjusting a gain. At first the reference point is corrected with using a first standard solution. To this end the first stardard solution is introduced in the vessel 3 and a potential difference corresponding to the known ion concentration of this first standard solution is detected. The level of measuring potential difference is so adjusted by means of the adjuster 6 that the indicator 5 indicates correctly the known ion concentration. Next the span is calibrated with the aid of a second standard solution having an ion concentration which is also known, but is different from the ion concentration of the first standard solution. To this end a potential difference corresponding to the known ion concentration of the second standard solution is detected and the gain is adjusted by the adjuster 6 so that the indicator 5 indicates correctly this known ion concentration. In this manner the reference point and span are calibrated. In general, the calibration of the reference point should be effected frequently, while that of the span need not be carried out during the measurement, because the reference point is affected by the drift, but the span is hardly influenced by the drift.
The known calibration method has a defect that the reference point and the span cannot be precisely corrected, because the reference point has to be calibrated before the span calibration and thus the latter calibration inevitably causes a deviation of the reference point and the calibration of span has to be effected on the basis of uncertained reference point which has not be corrected precisely. Moreover the adjustment of the adjuster 6 is quite troublesome, because it must be performed manually while observing an indications displayed on the indicator 5. This manual operation may be effected in an automatic manner, but the automatization naturally requires a very complicated construction and a large expense. Further, in the known calibration method during the measurement the adjuster 6 has to be adjusted sometimes with using both the first and second standard solutions so as to recalibrate the reference point. Therefore a relatively long time period is required for the recalibration during the measurement.